V G wrote:

> On Wed, May 25, 2011 at 6:36 PM, Gerhard Fiedler wrote:=20
>=20
>> I is only V/R for a linear R, or within a linear region of a
>> non-linear R. The C-E path of a bipolar transistor that is used as a
>> switch and is "on" is not usually in a linear region, differently
>> from a FET.
>=20
> From what I understand (I could be wrong), I =3D V/R for any point, as
> in, when a voltage V is applied across a resistance R, then the
> current flowing through R is equal to I =3D V/R.=20

Depending on what you consider "a resistance R", this could be correct
or not. It is correct for an ideal "Ohm" resistor, as it is the
definition of it. It is approximately correct for a real resistor as
long as you stay within its linear region. It is not correct for
anything else.

For example, take a normal 1/4 W metal film resistor of 1k. Apply
voltage across it, say 1V. You expect 1mA of current. That's what you
get -- approximately, as long as you don't look too close at noise, for
example (which is one divergence of real parts from Ohm's "law"). Then
increase the voltage. The current will follow as expected, but at some
point it will not anymore -- the resistor is heating up, and eventually
will burn, and the current will change drastically. That's where it came
out of its linear region. There are other effects besides exceeding the
max power spec, for example when exceeding the max voltage spec, and all
those max/min specs delimit the linear region of that resistor.

> Are you saying the internals of a BJT violate ohm's "law"? I mean, the
> violations of the law have been discussed on this list before, if I
> can remember correctly.=20

A bipolar transistor (and most other semiconductors) is no resistor, and
definitely no ideal "Ohm" resistor, so it can't really violate the "law"
-- it doesn't apply to it.=20

> I'm just not understanding the part about the linear region.

Under certain operating conditions, semiconductors can present a linear
dependency between current and voltage, and such regions may be called
"linear regions" (in this context). For example, the source-drain
connection of a MOSFET that's "on" has a region of drain-source voltage
where it behaves very close to a resistor.=20

Gerhard
--=20
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